J. Cent. South Univ. (2018) 25: 29−37 DOI: https://doi.org/10.1007/s11771-018-3714-y

Separation of azodicarbonamide from surface of

diatomite by froth flotation

ZHANG Qin(张覃)1, 2, 3, XIE Jun(谢俊)1, 2, 3, 4, CHEN Jian-hua(陈建华)5, CHENG Wei(程伟)1, 2, 3

1. Mining College, Guizhou University, Guiyang 550025, China; 2. Guizhou Key Lab of Comprehensive Utilization of Non-metallic Mineral Resources, Guizhou University,

Guiyang 550025, China; 3. Guizhou Engineering Lab of Effective Utilization of Regional Mineral Resources, Guizhou University,

Guiyang 550025, China; 4. College of Resources and Environment, Guizhou University, Guiyang 550025, China;

5. College of Resources and Metallurgy, Guangxi University, Nanning 530004, China

© Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: The separation of azodicarbonamide (AC) from the surface of diatomite by froth flotation is investigated in this research. Pure samples of diatomite, AC and 1:1 mixtures of the two were floated in a lab-scale flotation cell with collector dosage, frother type and dosage, and pH varied to determine the optimum experimental conditions. The diatomite sample and products from flotation tests were characterized using scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDX). The results of the flotation tests indicate that there is less AC on the surface of diatomite after flotation compared to the feed, while the AC present in diatomite pores remains unchanged. Additionally, Fourier transform infrared spectroscopy (FT-IR) was employed to study the mechanism of interaction between reagents and minerals. Key words: diatomite; azodicarbonamide; froth flotation Cite this article as: ZHANG Qin, XIE Jun, CHEN Jian-hua, CHENG Wei. Separation of azodicarbonamide from surface of diatomite by froth flotation [J]. Journal of Central South University, 2018, 25(1): 29–37. DOI: https://doi.org/ 10.1007/s11771-018-3714-y.

1 Introduction

Diatomite is a kind of siliceous sedimentary rock with a unique microporous structure. This structure consists of a large number of small holes and channels resulting in a high specific surface area [1]. As such, diatomite has many industrial uses including: heat preservation, filtration, packing, grinding, sodium silicate production, as a decoloring agent, as a filter aid, and as a catalyst carrier [2–6]. Diatomite, after modification, can

also be used as an effective absorption or filling material in a wider range of applications.

The molecular formula of azodicarbonamide (AC) is H2NCONNOCNH2. It is a pale yellow crystalline powder and is most widely used as a blowing agent in chemical foaming. This chemical can be used with polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), and acrylonitrile butadiene (ABS) resins to produce normal pressure foams, pressurized foams or delayed pressure foams [7, 8]. It is preferred for these applications due to its high volume, low price,

Foundation item: Project((2011)4012) supported by the High-Level of Innovative Talents of Guizhou Province, China Received date: 2016−05−10; Accepted date: 2017−12−01 Corresponding author: ZHANG Qin, PhD, Professor; Tel: +86–51–88292081; E-mail: zq6736@163.com; ORCID: 0000-0003-1253-

8652

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lack of toxicity, lack of odour, and absence of pollution problems associated with its use [7, 8]. Pure AC does have several disadvantages as a blowing agent including: a relatively high decomposition temperature, and difficulties in controlling decomposition rate and the deposition of decomposition products [9, 10].

Research on modified diatomite composite materials is ongoing [11]; however, there has been very little work to date on composite materials composed of diatomite filled with AC. A composite material of diatomite with AC filling the pores has a uniform hole-bubble structure, thus the blowing agent possesses the characteristics of both the diatomite and the AC. Additionally, the AC blowing agent alone can be prone to agglomeration during transportation and storage, so a composite material of diatomite filled with AC may help to address this issue. A major concern for diatomite-AC composites is that the properties of this material may be influenced by any AC remaining on the surface of the diatomite particles when diatomite and AC are mixed to obtain composite materials. It is therefore necessary to separate the AC from the surface of the diatomite without affecting the AC content of the diatomite pore structures.

The conventional purification methods for low grade diatomite include scrubbing, roasting and acid leaching [12–14]. Concentrates of diatomite produced using a scrubbing, sedimentation classification, and acid leaching process from a low grade diatomite ore (Jilin, China) had a chemical composition of 82.47% SiO2, 9.23% Al2O3 and 0.72% Fe2O3 [15]. Another process involving centrifugal separation was able to produce a diatomite concentrate with 73% yield and a grade of 86% SiO2, 4% Al2O3 and 1.9% Fe2O3 [16]. The operating conditions for this centrifugal separation were 1000 r/min centrifugal rotational speed, 12 L/min volumetric flow rate, and a 75 s feeding time, which resulted in a mass pull of 22% [16]. Conventional methods for diatomite purification can remove a portion of unwanted organic matter and clay minerals but fine gangue minerals will still report to the diatomite concentrate, affecting the quality of the final product. Conventional beneficiation methods are also restricted by long production cycles and high water, acid, and energy consumption [17]. Published research on the purification of diatomite by flotation is quite limited,

especially for the flotation separation of diatomite and AC. As flotation has proven useful in other applications in the rubber and plastic foaming agent industry [18], it may provide a new means of diatomite purification. Microflotation tests were employed in this work to investigate the influence of collector dosage, frother type and dosage, and pH on AC removal from the surface of diatomite by flotation. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were used to analyze the products of flotation. The relationship between mineral surface properties and floatability was investigated using Fourier transform infrared spectroscopy (FT-IR). 2 Materials and methods 2.1 Materials 2.1.1 Sample minerals preparation

Pure diatomite and AC samples used in this study were purchased from NCS Testing Technology Co., Ltd. (Beijing, China) without any further purification. The chemical composition of the pure diatomite is shown in Table 1. Purity of the AC is 97%.

The preparation method for the mixed mineral sample of diatomite and AC was as follows: diatomite, AC, ethanol, and deionized water were placed into an agate mortar, at a mass ratio of 1:1:1:1, slowly ground for 10 min, aged for 2 min, and then ground for another 10 min. After grinding, the powder was dried in air at room temperature and then preserved in a desiccator. The microstructures of the pure diatomite and the mixed mineral, as determined by SEM, are displayed in Figure 1. Note that in the mixed mineral, a portion of the AC was filled into the pore structure of the diatomite, while the remainder was adsorbed on the surface of the diatomite (Figure 1(b)). 2.1.2 Chemical reagents

Frothers used in flotation tests included: MC-1 Table 1 Chemical compositions of diatomite sample

(mass fraction, %)

Sample SiO2 Al2O3 Fe2O3 TiO2 CaO

Pure diatomite 96.40 0.03 0.12 0.01 0.11

Sample MgO K2O Na2O P2O5 L.O.I.a

Pure diatomite 0.05 0.02 0.03 0.02 2.79 a Loss on ignition

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Figure 1 SEM pictures of pure diatomite (a) and mixed

mineral sample (b)

(unsaturated polyol), ZX-2 (a mixture of several kinds of long carbon chain alcohols), BK201 (a mixture of several kinds of polyols). MC-1 was purchased from Beijing Chemical Reagent Company (Beijing, China). ZX-2 and BK201 were synthesized in the lab. The collector used was kerosene, purchased from Beijing Chemical Reagent Company (Beijing, China). Regulators used for pH control were NaOH and HCl purchased from Beijing Chemical Reagent Company (Beijing, China), both are of analytical grade. 2.2 Flotation tests

Microflotation tests were carried out in a XFG small flotation cell with a volume capacity of 40 mL. All feed samples were ground into a fine powder (100%m –75 µm) prior to flotation. The conditioning stage consisted of suspending 2.0 g of ore sample in 40 mL water at a mechanical stirring rate of 1980 r/min for 1 min followed by the addition of the collector (kerosene) and 2 min of additional conditioning. Finally, 20–200 mg/L of frother (MC-1, ZX-2 or BK201) was added and conditioned for 3 min with air pumped in 1 min after frother addition. The pH was adjusted by

diluted HCl and NaOH solutions. Froth was collected for 4 min. The microflotation concentrate and tails were each filtered, dried and weighed separately. 2.3 Ignition loss analysis

As shown in Figure 2, diatomite has a high thermal stability with minimal mass loss between 40 °C and 500 °C. The small amount of mass loss is mainly caused by the loss of adsorbed water and very little organic matter, while above 500 °C, the mass loss is due to the loss of the water of crystallization of minerals such as white mica and illite. It was determined that mass loss up to 300 °C is negligible for diatomite. AC decomposes at about 200 °C, with the decomposition equations as follows [19]:

2222 N+NCO2H→NCONNCOHH (1)

CO+NCONHH→NCO2H 222 (2)

322 NH+HNCO→NCONHH (3)

Ignition loss analysis was therefore used to

calculate the AC grade of each flotation product using a muffle furnace. Small amounts (0.2 g) of the microflotation concentrate and tails were placed in separate porcelain crucibles, kept in the muffle furnace at 300 °C for 30 min, and then cooled to room temperature and weighed. The AC content was then calculated from the ignition loss by using formula (4).

%1002.0

)AC( Ig

Lw (4) where w(AC) is the AC content and LIg is the ignition loss.

Figure 2 Results of thermo gravimetric analysis and

differential thermal analysis of diatomite

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2.4 SEM and EDX analysis In order to verify whether AC had adsorbed on

the surface of diatomite or filled in the pores, SEM and EDX analyses were employed. The original diatomite as well as flotation products was covered with gold by vapor deposition prior to being analyzed by SEM-EDX (Carl Zeiss-DSM630). 2.5 FT-IR analysis

In order to identify the mechanism of the interaction between the collector and the mineral particles, mineral samples conditioned with or without collector were analyzed by FT-IR using a Nicolet 6700 apparatus. Spectra were acquired in the range of 4000–500 cm–1 with 0.5 cm–1 resolution and processed using OMNIC ESP software. The solid samples were mixed with potassium bromide (KBr), ground and pressed into pellets for analysis. 3 Results and discussion 3.1 Effect of frother dosage

Tests were conducted to investigate the influence of different frother type and dosage on the flotation of diatomite and AC with the results shown in Figure 3. In this figure, the AC floating ratio is calculated by subtracting the recovery of diatomite from the recovery of AC.

Figure 3 Effect of frother dosage on floating ratio of

single material of diatomite and AC

It is clear that for these three frothers, the AC

floating ratio increases initially with increasing frother dosage and then decreases. For the MC-1 frother, the AC floating ratio increases significantly when the frother dosage increases from 20 mg/L to 60 mg/L, reaching the highest value of 89.4% at

60 mg/L. Similarly, when the dosage of ZX-2 increases from 20 mg/L to 80 mg/L, the AC floating ratio increases rapidly, peaking at 89.5% when the dosage of ZX-2 is 80 mg/L. After that, the AC floating ratio declines slightly with a further increase in frother dosage to a value of 85.5% at 200 mg/L. In the case of BK201, the AC floating ratio increases gradually when the frother dosage increases from 20 mg/L to 80 mg/L. When dosage of BK201 is 80 mg/L, the AC floating ratio is 80.15%.

The correlation between the AC floating ratio and frother dosage can be explained by a larger increase in AC flotation at low frother dosage and increased diatomite flotation recovery at higher frother dosages. It can therefore be concluded that MC-1 is the best frother for this system as it produced the highest AC floating ratio (an indication of selective flotation of AC from diatomite) at the lowest frother dosage. The optimum dosage of MC-1 in this system was determined to be 60 mg/L. 3.2 Effect of collector dosage

The effect of kerosene dosage on the flotation of the mixed mineral was evaluated for three different frother (MC-1) dosages of 20 mg/L, 60 mg/L and 100 mg/L. The AC recovery as a function of kerosene dosage is shown in Figure 4.

Figure 4 Effect of collector (kerosene) dosages on AC

recovery for the artificial mixed mineral

As can be seen from Figure 4, the flotation

recovery of AC increases with increased dosages of kerosene. Specifically, when the kerosene addition increases from 20 mg/L to 80 mg/L, the recovery of AC increases by 8%–10%, while further increases in kerosene dosage, from 80 mg/L to 160 mg/L,

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increases the recovery of AC by only 1%–2%. All three curves (MC-1 dosages of 20 mg/L,

60 mg/L and 100 mg/L respectively) display the same relationship between collector dosage and AC recovery, indicating that there is no significant interaction between the frother and the collector. The optimum dosage of kerosene in this system was therefore selected as 80 mg/L. 3.3 Effect of pH

As AC will decompose in strong alkaline condition, the effect of pH on flotation recovery was investigated in acidic and neutral conditions only. The flotation tests were carried out at two frother dosages (60 mg/L and 80 mg/L) and two collector dosages (60 mg/L and 80 mg/L). The results from these tests are shown in Figure 5.

Figure 5 Effect of pH on AC recovery for artificial

mixed mineral

As can be seen from Figure 5, the impact of

pH on the recovery of AC is significant. Each of the four different reagent combinations presents the same trend of increasing AC recoveries with increasing pH. The recovery of AC rises rapidly between pH 1 to 2, with more gradual increases from pH 2 to 5 with peak AC recovery achieved at pH 5. The explanation for the decrease in AC recovery from pH 5 to 7 may be that the diatomite and AC in the mixed mineral sample have different surface charges, resulting in an electrostatic attraction. This theory requires further verification. When comparing the curves of Figure 5, an increase of 20 mg/L in frother dosage appears to have a more significant influence on AC recoveries than a similar increase in collector dosage. The explanation for this may lay in the natural floatability difference between diatomite and AC. It

may be that frother plays a more important role than the collector in altering the floatabilities of AC and diatomite. 3.4 SEM and EDX analysis

In order to investigate the filling of the diatomite’s pores by AC as well as the removal of AC from the diatomite surface, SEM and EDX were used. A selection of SEM and EDX images for the mixed mineral feeds and microflotation products are presented in Figure 6.

As shown in Figure 6(a), for the 1:1 mixed mineral, it may be observed that diatomite contains AC both in the pores and on the surface prior to flotation. This is also supported by EDX analysis, with the results for the highlighted area shown in Table 2.

It was found that the surface of diatomite from the microflotation tails is very clean (Figure 6(b)). This indicates that most of the AC on the surface of the diatomite has been removed. In addition, EDX composition data from the selected region is not significantly different from the results for the mixed mineral feed, indicating that much of the AC in the diatomite pores is unaffected by flotation. The EDX compositional data for the selected region of the tails sample is shown in Table 2.

In Figure 6(c), it is clear that most of the AC has been removed from the surface of the diatomite, leaving only a few small fragments. The EDX measurement of the selected region of the flotation concentrate sample indicates that the N content of the sample is enriched compared with the mixed mineral prior to flotation. The complete EDX analysis of this region is shown in Table 2. The enrichment of N indicates that AC (which contains N), can be separated from diatomite by flotation. Determining the source of the floated AC (i.e., whether it is removed from the surface or the pores of the diatomite) requires further investigation. 3.5 FT-IR analysis

The results of FT-IR analysis of diatomite before and after treatment with kerosene are shown in Figure 7. The FT-IR peaks observed at 3698 cm–1and 3621 cm–1 were attributed to OH vibration associated with the hydroxyl groups of diatomite; bands at 3420 cm–1 and 1636 cm–1 were determined to correspond to the OH vibration of physically adsorbed H2O and the bending vibration of water

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Figure 6 SEM and EDX analysis of mixed mineral sample (a), tails sample (b) and concentrate sample (c) obtained with frother dosage of 60 mg/L and collector dosage of 80 mg/L at pH 5 molecules retained in the silica matrix of diatomite [20]. The peaks at 470 cm–1 and 1095 cm–1 may be attributed to the asymmetric stretching of the Si—O—Si bond [21]. All FT-IR spectra were

baseline corrected and normalized using the C—H peak at 2920 cm–1 which was consistent with time. As diatomite is formed from the accumulation of single-celled aquatic plants or diatoms, it may

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Table 2 Mass fractions of C, N, O, Al, Si, and Fe

measured by EDX analysis

Element Composition/%

Mixed sample Tails sample Concentrate sample

C 45.80 48.35 24.51

N 7.30 5.75 11.44

O 40.72 32.79 45.25

Al 0.71 1.60 3.18

Si 5.46 10.90 14.01

Fe 0.00 0.61 1.41

Figure 7 Infrared emission spectra of raw diatomite and

diatomite treated with kerosene

contain carbon [22].

For the sample treated with kerosene, no change was observed for one of the main characteristic peaks of crystalline quartz at 690 cm–1, suggesting that the framework of diatomite was undamaged by this reagent. As no new peaks are evident for the sample treated with kerosene, there is no evidence of any chemical reaction between diatomite and kerosene. 3.6 Flotation mechanism

FTIR analysis shows that there is not any chemical interaction between kerosene and diatomite, however, SEM-EDX and flotation tests results together show that AC on the surface was selectively floated up without affecting that in pores. During this process, AC, as a strong hydrophobic matter was floated up easily by kerosene after it has fell off from diatomite’s surface. Meanwhile, emulsion (kerosene+MC-1+water) happened under the effect of stirring and played as a depressant for diatomite [23, 24], and also as a hinder between

kerosene and AC in pores, thereby leaving AC filled diatomite in the pulp. A possible explanation for the depression process is thus proposed as follows: kerosene is immiscible in water, while the frother, containing both a hydrophilic polar group and a hydrophobic non-polar group, acts as an emulsifier [25]. The frother likely adsorbs onto the oil-water interface in this system, with its polar groups oriented towards the water phase and non-polar groups oriented towards the oil phase [26]. This adsorption of frother then forms a stable protective film on the surface of kerosene droplets. As a result, the kerosene can be easily dispersed in water as fine droplets to form an oil-in-water (O/W) emulsion [27]. In the flotation process, the emulsion forms a protective layer on the diatomite’s surface in areas where there is no AC adsorption, and once AC is removed the kerosene emulsion will occupy the vacated area and form a new protective layer. This protective layer will not only make diatomite surface hydrophilic but also will block a portion of the pores and prevent any contact with reagents or the pulp phase. However, the adsorption of the emulsion will also increase the diatomite’s hydrophobicity. Care must be taken to avoid excess emulsion adsorption on the diatomite surface, as this can reduce the floatability difference between the diatomite and AC. 4 Conclusions

As verified by flotation tests, SEM, and EDX analysis, an effective separation of AC from the surface of diatomite by froth flotation is feasible. At the optimum pH of 5, the frother and collector dosages required are 60 mg/L and 80 mg/L, respectively. Additionally, most of the AC on the diatomite surface can be removed without affecting the AC content of the pores. This result can be mainly attributed to interactions between the frother and the collector. Specifically, kerosene in this system is dispersed in the presence of frother to form an emulsion, which adsorbs on the diatomite surface to form a protective layer, preventing reagents and the pulp phase from interacting with the AC contained in the pores. This work provides a novel concept which may be applied for the removal of other harmful elements from the surface of filler materials.

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中文导读

泡沫浮选法分离硅藻土表面偶氮二甲酰胺 摘要：硅藻土是一种具有独特微孔结构的硅质沉积岩，可用作吸附或填充材料。偶氮二甲酰胺是工业

上广泛应用的一种发泡剂，将偶氮二甲酰胺填充到硅藻土孔隙中能改善硅藻土性能，使其同时具有硅

藻土和偶氮二甲酰胺特性，覆盖在硅 藻土表面的偶氮二甲酰胺会影响改性材料的性能。本文通过微

泡浮选对硅藻土表面分离偶氮二甲酰胺进行了研究，考察了捕收剂用量、起泡剂种类和用量、pH 值

对分离效果的影响，结果表明：当起泡剂 MC-1 用量为 60 mg/L，捕收剂煤油用量为 80 mg/L，pH 为

5 时，效果最好，偶氮二甲酰胺与硅藻土的上浮率之差达到 89.4%，偶氮二甲酰胺的回收率达到 72.5%。

通过扫描电镜和能谱对浮选产品进行表征，结果表明：与原矿相比，浮选后硅藻土表面偶氮二甲酰胺

大幅度减少，硅藻土孔隙中偶氮二甲酰胺基本不变。通过红外光谱研究了药剂与矿物之间的作用机理，

表明硅藻土与煤油之间不存在化学吸附，浮选体系中煤油与 MC-1 形成一种乳状液，吸附于硅藻土表

面形成保护层，防止药剂和矿浆与硅藻土孔隙内的 AC 相互作用。 关键词：硅藻土；偶氮二甲酰胺；泡沫浮选